1. Field of the Invention
The present invention relates to the field of data encoding and compression, and more particularly, to data encoding and decoding techniques and devices for interfacing data processing units.
2. Art Background
Analog transmission has dominated the telecommunications industry since its inception. Signals have been sent by some physical quantity (e.g. voltage) which are continuously varied as a function of time. With the advent of digital electronics and computers, it was soon realized that digital transmission is superior to analog transmission in several important ways. For example, analog circuits require amplifiers which attempt to compensate for the attenuation in the line. However, the analog systems cannot compensate exactly for the attenuation encountered, especially if the attenuation is different for different frequencies. Since the attenuation error is cumulative, long distance calls that go through many amplifiers are likely to suffer considerable distortion. In comparison, digital regenerators can restore the weakened incoming signal exactly to its original value, inasmuch as the only possible values are 0 and 1. Accordingly, digital regenerators do not suffer from cumulative attenuation error problems. In addition, digital transmission of voice, data, music, television, video, telephone and the like can all be multiplexed together to make more efficient use of the equipment. Very high data rates are possible using existing lines and the cost of digital computers and integrated circuits continues to drop.
When a telephone subscriber attached to a digital end office makes a call, the signal emerging from his local loop is an ordinary analog signal. This analog signal is then sampled and digitized at the end office by a CODEC (coder/decoder), producing a seven or eight bit number. Typically, the CODEC makes 8000 samples per second (at 125 microseconds per sample) because the Nyquist theorem provides that this is sufficient to capture all the information from a four kilohertz band width. This technique is commonly referred to as pulse code modulation (PCM).
A variety of protocol standards have been developed for telecommunication equipment to transfer data from one data processing unit to another. Consequently, there are now a variety of incompatible schemes in use around the world. One method that is in wide spread use is the Bell System T-1 carrier standard. As will be described, the T-1 carrier system can handle twenty-four voice channels multiplexed together. Typically, the analog signals are multiplexed using time division multiplex (TDM), with the resulting analog stream being fed to the CODEC rather than having twenty-four separate CODECs and then merging the digital output. Each of the twenty-four channels, in turn, is permitted to insert eight bits into the output stream. Seven of these bits are data, and one bit is for control, yielding fifty-six thousand bits per second (BPS) of data, and eight thousand BPS of signal information.
Under the T-1 standard, a frame consists of 24.times.8=192 bits, plus one extra bit for framing, yielding 193 bits every 125 microseconds. This gives a gross data rate for the T-1 system of 1.544 megabits per second (Mbps). The 193rd bit is used for frame synchronization. Normally, the receiver continously checks the frame bits to make sure that synchronization has not been lost. If the receiver gets out of sync, the receiver may then scan for the frame bit pattern to get re-synchronized.
The most recent Bell System T-1 standard (publication 62411, dated September, 1983) states that in order to insure adequate timing recovery of the regenerative digital facilities and circuit performance, certain pulse density requirements shall be met by data processing equipment coupled to the telecommunication system. The data processing equipment must not transmit more than fifteen logical zeros in a row, and in each and every time window of 8.times.(N+1) bits (where N can equal 1 through 23), there should be at least N logical ones present. In other words, a user desiring to transmit digital data over the Bell System phone lines must insure that the minimum number of logical ones is present in the data stream. For digitized voice transmission, the minimum number of ones may be easily provided by altering the least significant bit (LSB) (bit 8) for each channel of digitized voice. This minor modification to the digital voice sample would result in no perceptible difference in voice reconstruction by the receiving equipment. However, in the case where data is transmitted which does not represent digitized voice signals, but rather, bank accounts, spacecraft data, identification codes and the like, any alteration of the data through the insertion of ones in accordance with the Bell standard could prove disastrous.
As will be disclosed, the present invention provides methods and apparatus which permit a user to interface data terminal equipment for transmission and reception to the Bell T-1 carrier system. The present invention provides data encoding and compression techniques which insure transmitted data is compatible with the Bell T-1 standard, while preserving the integrity of the data at the receiving end after decoding.
Although the present invention is discussed in relation to the Bell T-1 carrier standard, it will be appreciated by one skilled in the art that the present invention has utility far exceeding the mere interface with the Bell telecommunication system. For example, the present invention may be used in a variety of applications where data encoding and compression with subsequent decoding after transmission is required.